1. Field of the Invention
The present invention relates to a sputtering target for fabricating a recording layer of an optical recording medium; a method of producing the sputtering target; a phase-change optical recording medium comprising a recording material in a recording layer thereof, which recording material is capable of causing changes in the phase thereof by the application of a light beam thereto, thereby recording, reproducing and overwriting information therein; a method of forming the recording layer for the above-mentioned phase-change optical recording medium by using the above-mentioned sputtering target; and an optical recording method using the above-mentioned phase-change optical recording medium which is suitable for the application of optical memory devices, in particular, rewritable compact disk (CD-rewritable) application.
2. Discussion of Background
There is conventionally known a phase-change optical information recording medium which utilizes phase changes between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase as one of the optical recording media which are capable of recording, reproducing and erasing information by the application thereto of electromagnetic waves, such as a laser beam. This kind of phase-change optical information recording medium enables the overwriting of information by the application of a single laser beam thereto, although such overwriting is difficult to conduct by the application of a single laser beam in magneto-optical memory using a magneto-optical recording medium. An optical system of a drive unit for the phase-change type optical information recording medium can be designed simpler than that for a magneto-optical recording medium, so that recently research and development of this kind of recording medium has been actively conducted.
As disclosed in U.S. Pat. No. 3,530,441, the so-called chalcogen-based alloys, such as Ge--Te, Ge--Te--Sn, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te and Se--As, are conventionally used as recording materials for the phase-change optical recording medium. In addition, it is proposed to add an element of Au to the above-mentioned Ge--Te based alloy to improve the stability and to increase the rate of crystallization of the recording material as disclosed in Japanese Laid-Open Patent Application 61-219692. Furthermore, the addition of Sn and Au to the Ge--Te based alloy, and the addition of Pd to the same are respectively proposed in Japanese Laid-Open Patent Applications 61-270190 and 62-19490 for the same purposes as mentioned above. Furthermore, recording materials comprising a mixture of Ge, Te, Se and Sb, and a mixture of Ge, Te and Sb are respectively disclosed in Japanese Laid-Open Patent Applications 62-73438 and 63-228433, each of which has specific composition ratios of constituent elements of the recording material for the improvement of the recording and erasing repetition properties of the obtained recording medium.
However, none of the above-mentioned conventional phase-change optical recording media satisfies all the requirements for the phase-change rewritable optical recording medium.
Japanese Laid-Open Patent Application 63-251290 discloses an optical information recording medium which comprises a recording layer comprising a compound of a multi-component system composed of substantially three or more components in a single crystalline phase. The single crystalline phase of the compound of a multi-component system composed of substantially three or more components is considered to contain a compound with a stoichiometric composition, for example, In.sub.3 SbTe.sub.2, in the recording layer in an amount of 90 atom % or more. It is mentioned that recording and erasing characteristics can be improved to some extent by the provision of this kind of recording layer. However, this optical information recording medium has the shortcomings that the erasability is low and the laser power required for recording and erasing cannot be sufficiently reduced.
Furthermore, Japanese Laid-Open Patent Application 1-277338 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by the formula of (Sb.sub.a Te.sub.1-a).sub.1-b M.sub.b, wherein 0.4.ltoreq.a&lt;0.7, b.ltoreq.0.2, and M is one element selected from the group consisting of Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn. The basic system of the aforementioned alloy is Sb.sub.2 Te.sub.3, and the addition of a large excess of Sb to this composition in terms of atomic percentage enables high-speed erasing operation and improves the repetition properties. The addition of the element M can further enhance the high-speed erasing performance. In addition to the above advantages, this reference asserts that the erasability by the application of DC light is increased. However, this reference does not show any specific erasability obtained at the overwriting operation, and according to the experiments conducted by the inventors of the present invention, the erasability in the course of the overwriting operation is unsatisfactory and the recording sensitivity is insufficient for use in practice.
Japanese Laid-Open Patent Application 60-177446 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by the formula of (In.sub.1-x Sb.sub.x).sub.1-y M.sub.y, wherein 0.55.ltoreq.x.ltoreq.0.80, 0.ltoreq.y.ltoreq.0.20, and M is one element selected from the group consisting of Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se and Bi. In addition, Japanese Laid-Open Patent Application 63-228433 discloses a recording layer of an optical recording medium, which comprises an alloy with a composition of GeTe--Sb.sub.2 Te.sub.3 --Sb (excess). The sensitivity and erasability required for the recording medium cannot be satisfied in any of the above-mentioned conventional optical recording media.
In addition, a recording layer of an optical recording medium is made of a Te--Ge--Sb alloy, with the addition thereto of nitrogen atom, as disclosed in Japanese Laid-Open Patent Application 4-163839; a recording layer is made of a Te--Ge--Se alloy, with the addition thereto of a nitride comprising at least one element of Te, Ge or Se, as disclosed in Japanese Laid-Open Patent Application 4-52188; and a recording layer is made of a Te--Ge--Se alloy, with nitrogen atom being adsorbed thereby.
However, there are still some problems remaining unsolved in those conventional optical recording media.
As previously mentioned, most important points to be improved in the conventional phase-change optical recording media are to enhance the recording sensitivity and erasing sensitivity, to prevent the decrease of the erasability in the course of the overwriting operation, which is caused by the portions remaining unerased, and to increase the life of a recorded portion and that of a non-recorded portion.
With the rapid spread of a compact disk (CD), a write-once compact disk (CD-R) capable of writing data therein only once has been developed and put on the market. However, if the user does not succeed in writing the data in the CD-R, the CD-R becomes unavailable because it is impossible to correct the data. To compensate such a shortcoming of the CD-R, a rewritable compact disk is eagerly expected to be put to practical use.
A rewritable compact disk has been researched and developed by utilizing the magneto-optical recording medium, but the compact disk thus obtained has the shortcomings that there is difficulty in satisfactorily carrying out the overwriting operation, and the compatibility with the CD-ROM or CD-R is poor. Since it is considered that a phase-change optical recording medium can ensure the compatibility with the CD-ROM or CD-R more favorably in principle when compared with the above-mentioned rewritable compact disk of a magneto-optical type, the application of the phase-change optical recording medium to the rewritable compact disk has been actively researched and developed recent years.
Such a rewritable compact disk obtained from the phase-change optical recording medium is reported in some references, for instance, "Proceedings of the 4th Symposium on Phase-Change Recording" p.70 (1992), Furuya et al.; "Proceedings of the 4th Symposium on Phase-Change Recording" P.76 (1992), Jinno et al.; "Proceedings of the 4th Symposium on Phase-Change Recording" p.82 (1992), Kawanishi et al.; Jpn. J. Appl. Phys. 32 (1993) p.5226, T. Handa et al.; "Proceedings of the 5th Symposium on Phase-Change Recording" p.9 (1993), Yoneda et al.; and "Proceedings of the 5th Symposium on Phase-Change Recording" p.5 (1993), Tominaga et al. However, none of the rewritable compact disks reported in the aforementioned references is satisfactory with respect to the overall performance, such as the compatibility with CD-R, recording and erasing performance, recording sensitivity, maximum number of repeated overwriting operations, maximum number of repeated reproducing operations, and shelf stability. Those drawbacks are mainly ascribed to low erasability due to the composition and configuration of a recording material employed in each compact disk.
Under such present conditions, there is increasing a demand for development of a phase-change recording material with high erasability, and high recording and erasing sensitivities, and in addition, a phase-change rewritable compact disk capable of exhibiting excellent overall performance.
To meet the aforementioned demand, the inventors of the present invention have proposed Ag--In--Sb--Te based recording materials, for example, as disclosed in Japanese Laid-Open Patent Applications 4-78031 and 4-123551; Jpn. J. Appl. Phys. 31 (1992) 461, H. Iwasaki et al.; "Proceedings of the 3rd Symposium on Phase-Change Recording" p.102 (1991), Ide et al.; and Jpn. J. Appl. Phys. 32 (1993) 5241, H. Iwasaki et al.
It became apparent that the phase-change optical disk thus obtained exhibited excellent performance. However, still more endeavors should be made to produce a phase-change optical recording disk which is capable of securely ensuring the compatibility with the CD-R, and perfectly satisfying the previously mentioned overall performance required for the rewritable compact disk, and to establish the method of producing a phase-change optical disk capable of forming a new market.